Implement Check Phase Ruleset

[EttoreM]

This PR addresses #45

Implemented rules

#	Rule	sh:severity
1	RootDataEntity --> mentions SHOULD reference the CheckValue entity	sh:Warning
2	The CheckValue entity MUST have additionalType with @id of https://w3id.org/shp#CheckValue (i)	sh:Violation
3	CheckValue entity MUST be of type AssessAction	sh: Violation
4	The name of the CheckValue entity MUST provide a human-readable summary of the review and result (ii)	sh:Violation
5	CheckValue --> object SHOULD point to the root of the RO-Crate.	sh:Warning
6	CheckValue --> instrument SHOULD point to an entity typed schema:DefinedTerm	sh:Warning
7	CheckValue entity MAY have startTime property if action began	sh:Info
8	CheckValue --> startTime MUST follows the RFC 3339 standard.	sh:Violation
9	CheckValue entity SHOULD have endTime property if action ended	sh:Warning
10	CheckValue --> endTime MUST follows the RFC 3339 standard.	sh:Violation
11	CheckValue entity SHOULD have actionStatus property	sh:Warning
12	CheckValue --> actionStatus MUST have one of the allowed values	sh:Violation
13	CheckValue --> agent SHOULD reference the agent who initiated the check (iii)	sh:Warning

(i) Rule 2 is not an actual rule, but rather a definition of what constitutess a CheckPhase object. Hence it is not possible to code it, given its axiomatic nature.
(ii) This cannot be entirely done without a manual check. Here we only check that the name property of the Check Phase object is a string of at least 20 characters.
(iii) All we are enforcing / testing here is that checkValue has a property agent which points to an IRI.

---

Rules not implemented (they need some clarification)

#	Rule	sh:severity
1	Submitted crate SHOULD be checked for integrity and completeness against the BagIt payload manifest and tag manifest, considering at least the SHA-512 algorithm.	sh:Warning
2	This phase MAY also check any cryptographic signatures.	sh:Info